The invention relates generally to a method and apparatus for positioning a web of material which is to be transported in a stepwise manner by means of at least two forward feed means through processing stations disposed between the feed means. Such a procedure is involved for example in printing on webs of material which are fed through printing stations and possibility for separating the individual printed images or areas produced on the web of material, for example by means of a stamping operation.
In one form of such a method, the length of the distance to be covered by the web of material during one transportation step is stored in a programmable computer which is operable to control the forward feed means. Thus the effective length of the distances to be covered in each transportation step by the web of material are detected by way of an opto-electronic detector arrangement and compared as an actual value to the reference value stored in the computer. In the event of a deviation between the actual value and the reference value, a correction operation is carried out in order to match the actual value to the reference value, independently of matching or register marks which may be provided on the web of material. However, such a method is frequently no longer adequate for the levels of precision which are required nowadays and which require a degree of accuracy of less than 0.1 mm, possibly even only 0.01 mm, with a register length, that is to say the distance to be covered by the web of material in each transportation step, for example of 1 m.
An essential requirement in regard to attaining the aboveindicated level of accuracy required is that the tension of the web of material in the portion thereof which is between the first and second forward feed means is to be as constant as possible in order to minimise variations in the length of that portion of material, due to shrinkage or elongation thereof. As will be appreciated, such variations in length will affect the length of the printed image or pattern applied to the web of material. It will be further appreciated that when printing is applied to the web in a plurality of printing operations in a corresponding number of printing stations in which for example printed images of different colours are successively applied to the web, thus making up the complete printed image, it is also necessary to ensure that each portion of the web to which printing is to be applied is of the same length in all the successive printing operations or printing stations, in order to achieve the required level of accuracy. That same consideration also applies in regard to a severing or cutting station if such is provided, in which the printed portion of web is cut away from the remainder of the web, for example by means of stamping blades. If the above-indicated requirement is not met, the position of the actual severing cut which severs the printed portion of the web from the remainder thereof would deviate at least in part from the desired or reference position thereof.